First scientists discovered that the milder winters were shrinking the sheep in the Outer Hebrides at a rate of 3.5 ounces (100g) a year as smaller, weaker lambs were surviving in the warmer weather.

Now they have discovered the same process is turning the dark coats of Soay sheep on Hirta in the St Kilda archipelago lighter.

On the isolated windswept island in the Outer Hebrides about three quarters of sheep have dark brown coats, while the remainder have light sandy ones.

But despite the fact that the dark-coated Soay sheep are larger, which is usually linked to survival and reproductive success, the frequency of light-coated sheep has increased over the last 20 years, while the climate has warmed in the North Atlantic.

Dr Shane Maloney, an animal researcher whose findings are published in the Royal Society journal Biology Letters, said: "If environmental effects are the cause of the decline, then we can expect the proportion of dark coloured Soay sheep to decrease further."

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He said in colder environments, mammals with darker coats absorb more solar radiation and so need to expend less food energy keeping warm than do their lighter counterparts.

When the climate warms, as is occurring worldwide, the advantages of darker coats in colder environments will wane, leading the number of dark coloured sheep to fall, and to reduce further as the weather trend continues.

Dr Maloney, of the University of Western Australia, said: "Our finding that the proportion of dark-coloured Soay sheep decreased over the past 20 years as ambient temperature increased could be interpreted in several ways, the most parsimonious being that dark colouration has provided an energetic advantage in winter that is being attenuated in a warming climate.

"Our finding that colouration is associated with winter temperature the previous year suggests that it is the differential survival through winter to the subsequent breeding season and the breeding success the subsequent year that provides the mechanism for colour selection.

"With warming winters, more of the lighter type is apparently surviving to reproduce."

A separate study published earlier this month found the Soay sheep have shrunk by five per cent in size over the past quarter of a century due to a change in climate.

Similar effects have been observed in fish, including cod, salmon and sticklebacks, marine iguanas, large-horned Canadian sheep, North American squirrels and blue tits.

Added Dr Maloney: "The warming climate at St Kilda could have weakened a selection pressure favouring larger Soay sheep, historically advantaged in the cold, or perhaps augmented selection for smaller variants as resources have become more limited with the expanding population."